Heating pads and electric blankets are devices used to keep an object warmer than a surrounding temperature. For instance, they may be used to keep a person warm in a bed, or to warm a limb (e.g., an electric mitten), an animal (e.g., an electric pet blanket), an object (e.g., a pipe heater to thaw a pipe or prevent a pipe from freezing), etc. Heating pads and electric blankets in general will be referred herein as “heating pads,” unless the circumstances clearly indicate otherwise. Additional layers of insulation may be used with a heating pad, such as an outer layer of insulation to lessen heat loss, or an inner partially-insulative layer to lessen a risk from a hot spot in the heating pad excessively heating an adjacent portion of the object. The additional layers of insulation may be included with the heating pad, or may be external to the heating pad (e.g., an ordinary bed blanket, comforter, or the like), spread over at least a portion of the heating pad.
Electric heating pads and blankets have heating cables that include electrical conductor(s) or wire(s) as a heating element. A conventional heating cable has one heating conductor or wire. More advanced heating cables could have more conductors which could be used as heating wires or signal sensing wires. The electrical conductors commonly are wound in a helical shape along the length of the heating cable, in order to increase the length of the conductors per unit length of the heating cable, and to provide more even heating circumferentially around the heating cable. However, other configurations of one or more of the electrical conductors may be used.
For a cable with multiple helical wound conductors, the conductors are disposed substantially coaxially along the length of the heating cable. The inner conductor can be wound around a dielectric core which may also be used to produce a desired amount of stiffness or flexibility to the cable. A sheath of a resistive material used as a separation layer is disposed around the inner conductor, and the outer conductor is wound around the separation layer. A thermally conductive outer sheath is disposed around the outer conductor to protect the heating cable while permitting heat to pass to other portions of the heating pad. For cables that use one or multiple conductors for signal sensing, the outer conductor is normally used as a heating element, but the disclosure is not limited in this regard. Electricity passes through the heating element, and the inner conductor is used as a sensing wire.
The power dissipated in the electrical conductor varies with the resistance of the electrical conductor, as well as the current (or voltage) through the electrical conductor. The electrical conductors are commonly made from a material that has a positive temperature coefficient (“PTC”) characteristic, in which the resistance of the wire increases with an increasing temperature over a temperature range of interest.
The heat produced by the electrical conductors also will increase the temperature of the resistive material, producing a change in resistance of the separation layer with a change in its temperature. The separation layer may exhibit a negative temperature coefficient (“NTC”) characteristic in which the resistance of the separation layer decreases as its temperature increases over a temperature range of interest.
Temperature control methods known in the art for heating pads and electric blankets include using a conductor or wire that provides a feedback signal to a control for monitoring temperature and detecting local hot spots. A conductor is coupled to a control circuit, and the circuit is designed to provide a phase change (i.e., a phase shift) with a change in the temperature of the wire. This phase shift is used as an indicator of the temperature of the wire. Another control method known in the art provides hot spot detection by using an NTC resistive material. Limited control can be accomplished by detection of a low-resistance path at a hot spot between heating and sensing wires. When the resistance is lower than a pre-set threshold, the circuit will shut down power to prevent over heating.
A drawback of the conventional approaches is that the precision of the temperature control is limited by the sensitivity of the temperature-sensing material or the method of processing feedback provided from the temperature-sensing material. The sensitivity may be low, and furthermore the sensitivity may vary over at least a portion of the temperature range of interest. Over at least a portion of the temperature range of interest, the sensitivity may not be adequate to provide a desired accuracy of temperature control. Furthermore, known control algorithms may be susceptible to degraded accuracy under a variety of conditions, such as the heating pad being partially covered, uncovered, folded over, etc.